This invention relates to data transmission systems for transmitting and receiving data frames among a plurality of nodes through transmission lines, by which each frame includes a portion for indicating an occurrence of abnormality.
FIG. 8 is a block diagram showing the structure of a conventional data transmission system. A receiving node 101, for example, receives the data transmitted from a transmitting node 102 through a transmission line 104. A sensor 401 supplies the information to be transmitted to the transmitting node 102. The data is transmitted through the transmission line 104 being divided in a plurality of frames 501 through 504 501. FIG. 9 shows the format of a transmission frame of FIG. 8. The frame includes a frame information 601, data 602 (602-1, 602-2, -- ), abnormality judgment data 603, and a check code 604 (e.g., cyclic redundancy check code or parity check code).
The operation of the data transmission system of FIG. 8 is as follows. In FIG. 8, the transmitting node 102 transmits the data supplied from the sensor 401. The data is transmitted successively divided in a plurality of frames 501 through 504 501. The transmitting node 102 increments the value of the abnormality judgment data 603 each time a frame is transmitted. Upon receiving a frame, the receiving node 101 confirms the value of the abnormality judgment data 603 of the frame. If the value of the abnormality judgment data 603 is not updated (i.e., incremented), the receiving node 101 judges that some abnormality has occurred and the frame is abnormal.
Upon detecting an abnormality of the sensor 401, the transmitting node 102 transmits the frame without incrementing the value of the abnormality judgment data 603 since such sensor data is not useful for the control operation at the receiving node 101. The receiving node 101 judges that a frame is abnormal when the abnormality judgment data 603 thereof is not updated. When the frames having an un-updated abnormality judgment data 603 are received successively, the receiving node 101 judges that either the transmitting node 102 or the sensor 401 is abnormal.
By the way, FIG. 8 shows only one sensor 401 which supplies information. A plurality of sensors and actuators, however, supply information transmitted from a transmitting node to a receiving node 101.
The above conventional data transmission system, however, has the following disadvantage. Although the abnormality of the transmitting node and the information supply means (sensors and actuators) can be detected at the receiving node, the abnormality of the transmitting node and that of the information supply means cannot be distinguished from each other. The receiving node judges that the whole transmission frame is abnormal even when the transmitting node is normal and only a part of the data supplied from the information supply means is abnormal. Thus the data supplied from normal sensors, etc., are discarded and cannot be utilized at the receiving node.